There is nothing that history does better than to throw a spanner in the works of modern policy. Forgive the rhetorical flourish but Beijing’s attempts to warm its oft-recently cool relations with its frenemy, Tokyo, in this 40th anniversary year of the establishment of diplomatic relations between China and Japan, have been brought to a grinding halt by the 1937 Nanjing Massacre.

On Monday, the mayor of the Japanese city of Nagoya, greeting a visiting delegation from Nanjing, denied that the massacre had taken place. The reaction of China’s netizens to this mendacity–and the Nanjing delegation’s apparent lack of a sufficiently outraged response–was so intense that the issue became front page news even in Chinese state media. The Nanjing city government has now suspended its sister-city relations with Nagoya and China’s foreign ministry has had to weigh in, expressing a finely weighted “strong dissatisfaction”.

Tokyo, too, is trying to distance itself from the dispute. Chief Cabinet Secretary Osamu Fujimura said that Nagoya and Nanjing should settle it themselves. “It isn’t a matter for the state to interfere in as they have sister-city relations,” he said.

This particular interruption to national relations may blow over quickly, at least as a diplomatic dispute. Yet it is a reminder of how easy it is to unleash the historical animosity between the two countries, forces that in China can burst out in sometimes violent and often unexpected ways, regardless of what governments would wish. And it will give Tokyo yet another reason to be wary of Beijing’s rapprochement.